Voltage in Electrical Systems 1.3.1

Objectives Explain the similarities and differences between Newton’s law of universal gravitation and Coulomb’s law. Explain how force between two like charges and the force between two unlike charges are different. Describe how to create an electric field and interpret the information given in a drawing of an electric field.

Universal Forces Include: gravitational and electrical Gravitational force acts between two or more masses Electrical force acts between two or more charges Called universal because each force behaves the same anywhere in the observable universe. The two are field forces, act over a distance.

Gravitational Force Why cover gravitational force? 17th century, Isaac Newton. Newton’s universal law of gravitation Every object in the universe attracts every other object with a force that is directly proportional to the mass of each body and inversely proportional to the square of the distance between them.

Gravitational Force G = universal gravitational constant = 6.67 x 10-11 N•m2/kg2 m1 = mass of first body m2 = mass of second body d = distance between the two bodies

Electric Charge Electrostatic forces, the comb and the CRT. Charge – property of and object that causes electrical force. Two types of charge: positive and negative Electrical forces are either attractive or repulsive. Like charges repel Opposite charges attract.

Origin of Charge Structure of atom. Charge of electron is equal in magnitude to proton but opposite in sign. Normal atom is neutral because number of protons and electrons are equal.

Origin of Charge Charge can transferred Net Charge = #protons – #electrons Charge can transferred Comb example Principle of conservation of charge – net charge in an isolated system never changes.

Electrical Force In 18th century, French scientist Charles Coulomb discovered the relationship between force, charge and distance. Coulomb’s law The electrical force between two charged bodies is directly proportional to the charge on each body and inversely proportional to the square of the distance between them.

Electrical Force SI unit for charge is the Coulomb (C). Elementary charge of one electron or proton is 1.60 x 10-19 C q1 and q2 are the charges on two objects. d = distance between charged objects K = constant = 9.0 x 109 N•m2/C2

Electrical Force Coulomb’s law similar to Newton’s universal law of gravitation. But, gravitational force is always attractive. Direction for electrical force depends on charge of particles.

Scale and Universal Forces Small distance and mass as in atoms, electrical forces are important and gravitational force is insignificant. Large distance and mass, significance reverses. Thus Electrical forces govern the structure of atoms, molecules, solids, liquids and gases. Gravitational forces govern the structure of planets, stars, galaxies and the universe.

Gravitational and Electrical Fields Field forces are alterations in space around the body creating the field. They are models used by scientists to help them understand and predict how forces are transmitted from one object to another. The field forces are vector quantities.

Gravitational Field Electrical Field What happens when you substitute the respective laws into each equation above? E does not depend on size of test charge. g does not depend on size of test mass.

Field Line Diagrams Illustration of a field can be done with field lines. Direction of field at any given point is tangent. Lines are close, field is strong Lines are far apart, field is weak